Correlation between vibrational anomalies and emergent anharmonicity of local potential energy landscape in metallic glasses

TL;DR

This study reveals a strong link between vibrational anomalies and anharmonicity in metallic glasses, challenging the harmonic disorder paradigm by showing that the boson peak correlates with anharmonic features of the potential energy landscape.

Contribution

The paper provides atomistic evidence connecting the boson peak to anharmonicity and the topology of the potential energy landscape in metallic glasses, proposing a new paradigm.

Findings

Boson peak intensity inversely proportional to activation energy in PEL

Larger boson peak associated with shallower basins and lower barriers

Atomic vibrations at the boson peak are strongly anharmonic

Abstract

The boson peak (BP) is a universal feature in the Raman and inelastic scattering spectra of both disordered and crystalline materials. The current paradigm presents the boson peak as the result of a Ioffe-Regel crossover between ballistic (phonon) and diffusive-type excitations, where the loss of coherence of phonons is described as a purely harmonic process due to structural disorder. This "harmonic disorder" paradigm for the BP has never been challenged or tested at the atomistic level. Here, through a set of atomistically-resolved characterizations of amorphous metallic alloys, we uncover a robust inverse proportionality between the intensity of boson peak and the activation energy of excitations in the potential energy landscape (PEL). Larger boson peak is linked with shallower basins and lower activation barriers and, consequently, with strongly anharmonic sectors of the PEL. Numerical evidence from atomistic simulations indicates that THz atomic vibrations contributing the most to the BP in atomic glasses are strongly anharmonic, as evidenced through very large values of the atomic- and mode-resolved Gr\"{u}neisen parameter found for the atomic vibrations that constitute the BP. These results provide a direct bridge between the vibrational spectrum and the topology of the PEL in solids, and point towards a new "giant anharmonicity" paradigm for both generic disordered materials and for the phonon-glass problem in emerging materials for energy applications. In this sense, disorder and anharmonicity emerge as the two sides of the same coin.